So, I imagine that a lot of PCs ride around on horses at least some of the time. They are great for quicker long-distance travel, and make excellent platforms to fight from. But, when was the last time one of those fantastic horses stepped in a hole or threw a shoe? Horses shouldn’t just be a maintenance-free battle car, there is a reason that only nobles kept horses in the time period most games take place in. Horses require care, lots of it.
First, the food. Horses need to eat a lot, and frankly, the grass growing around the roadside just isn’t that great of quality. Sure, it can keep the beast going through the day, but he also needs some real nutrition to maintain fighting trim. Oats are called for, or some other “supplement” feed. Every day.
Grooming is also rather important to keeping horses happy and healthy. Removing parasites, detangling matted fur/manes, checking for sores under the saddle and straps, each should be done every day to prevent disease and pain in your beast of +1 fast travel.
Now for the meat of the matter. Riding through difficult terrain. Taking your horse “off-road” may seem like it would be a natural thing to do, after all, horses are animals, and animals wander all over the place, right? True, animals go everywhere, but an animal is really suited to one kind of environment, and horses are great on the steppe. Or they were, when they were smaller, more agile, and used to dealing with untamed grassy flatland every single day of their lives. But nowadays, and in the days of your PCs as well, horses are bred, kept, trained, and pampered to maximize their usefulness to the human population. Warhorses are heavy and powerful to cause as much damage as possible on the battlefield, riding horses are lighter (so they eat less), but strong enough to carry a person. Draft horses are used for carrying or pulling loads along roads. And each of them are raised in paddocks and trotted out to the pasture for training and exercise.
What happens when you take an animal that has known prepared surfaces all it’s life, and then make it run through wild forestland, over rocks, up mountains, across plowed fields and through deserts? It’s not going to go so well. Walking is one thing, of course, but running? That’s a thousand pounds of horsemeat hurtling along at 25 mph on spindly little legs with not much time to look where each one is being placed. There’s a pretty decent chance something bad is going to happen to a horse being put through that obstacle course. Which is where the following table comes in.
For each minute spent travel at above a trot (The four speeds of horses being “Walk” “Trot” “Canter” and “Gallop”), roll a percent chance according to the terrain type. If the chance for a mishap is rolled, then roll a d20 to see what kind of mishap occurs. Feel free to adjust DCs and checks to suit your game.
Galloping through the woods is not something that is not normally done, and it should feel just as risky as it is in real life. Please note, however, that galloping on a road is a different matter, as those surfaces are “prepared” in that they are generally well suited to foot and wheeled traffic. Horses are used to roads, that’s why roads exist. If it’s raining or the roads are otherwise less suited to traffic than normal, you can roll for the cropland mishap.